1. Field of the Invention
This invention relates generally to the field of magnetron sputtering and more specifically to planar magnetron sputtering apparatus for sputtering magnetic target materials.
2. Background of the Invention
Glow discharge sputtering is a well-known process that is widely used to deposit thin films of various kinds of ceramic and metallic materials onto the surfaces of objects. For example, glow discharge sputtering is commonly used in the electronics industry to produce integrated circuit semiconductors and photovoltaic cells, as well as the magnetic tapes and disks used in audio, video, and computer applications. Sputtering is also used to deposit coatings on architectural glass, computer screens, sheet metal, sunglasses, automobile parts, automobile glazing, surgical implants, jewelry, tool bits, sheet plastic, fabrics, and fiber optics, just to name a few.
One type of glow discharge sputtering is diode sputtering. Diode sputtering is usually conducted in a vacuum chamber and in the presence of an inert sputtering gas, such as argon, that is maintained under very low pressure. The material to be sputtered (usually referred to as the target) is connected to the negative terminal of a DC power supply and serves as a cathode. The positive terminal of the power supply may be connected to a separate anode structure or to the vacuum chamber itself, depending on the application. The strong electric field between the target/cathode and the anode ionizes the sputtering gas, producing a glow discharge. Since the target/cathode is held at a strong negative potential, the positive ions from the glow discharge bombard the target material and eject target atoms, which then deposit on a work-piece or a substrate placed generally in line of sight of the target. Unfortunately, however, the diode sputtering process is slow and relatively inefficient compared to other film deposition techniques.
The efficiency of the diode sputtering process has been significantly increased by using a magnetic field to confine the glow discharge to the immediate vicinity of the target surface. Basically, while the sputtering yield (i.e., the number of target atoms dislodged or sputtered per incident ion) depends on the energies of the incident ions, the overall sputtering rate depends on both the energies of the incident ions as well as the total number of ions that bombard the target surface during a given time. Therefore, the sputtering rate can be increased by using a magnetic field to confine the ions and electrons produced in the glow discharge to the region immediately adjacent the surface of the target. The presence of such a plasma-confining magnetic field also has other benefits, such as allowing the sputtering operation to be conducted at lower gas pressures, confining the glow discharge to the neighborhood of the electrodes, and reducing electron bombardment of the substrates.
A common type of magnetic sputtering device is the planar magnetron, so named because the target is in the form of a flat circular or rectangular plate. Powerful magnets placed behind the target plate produce a strong plasma-confining magnetic field adjacent the front surface of the target, thus greatly increasing sputtering .efficiency. While numerous shapes and configurations of plasma-confining magnetic fields exist, it is common to shape the plasma-confining magnetic field so that it forms a closed loop ring or "racetrack" over the surface of the target material. When viewed in cross-section, the flux lines of the magnetic field loop or arch over the surface of the target, forming a magnetic "tunnel," which confines the glow discharge to a ring or racetrack shaped sputtering region next to the front surface of the target. As is well-known, the electric field created by the high voltage between an anode and the target/cathode acting in combination with the closed loop magnetic field causes electrons within the glow discharge to gain a net velocity along the racetrack, with the magnitude and direction of the electron velocity vector being given by the vector cross product of the electric field vector E and the magnetic field vector B (known as the E.times.B velocity). The shape of the predominate electron path defines the portion of the target material that will be sputtered.
While planar magnetrons are widely used to sputter non-magnetic target materials, such as aluminum and its alloys, they have not proven particularly useful for sputtering magnetic materials such as, for example, iron, nickel, iron-nickel alloys, and cobalt-chromium alloys. Simply replacing a non-magnetic target in a planar magnetron with a ferromagnetic target of the same general configuration usually causes most, if not all, of the magnetic field to be shunted through the magnetic target. This reduces the intensity of the plasma-confining magnetic tunnel above the target to the point where it can no longer effectively confine the plasma over the surface of the target, thus reducing planar magnetron sputtering to that of ordinary diode sputtering with its attendant relatively slow sputter rate and inefficiency.
One solution to the problem of sputtering magnetic target materials has been to use very thin targets, so that the target does not short out the entire magnetic field. If the target is thin enough (approximately 2-3 mm for a highly ferrous material), sufficient excess magnetic flux will remain over the front surface of the target to produce a plasma-confining magnetic tunnel. Unfortunately, however, such thin targets are rapidly depleted, thus requiring frequent replacement and substantial downtime of the sputtering apparatus.
Another solution to the problem has been to strengthen the magnetic field so that it can saturate thicker targets, yet still produce a plasma-confining magnetic tunnel over the front surface of the target. Usually a field strength in the range of about 80-100 gauss in a direction parallel to the target surface is required to achieve the magnetron effect. While stronger magnets are more expensive, they can, at least theoretically, result in a magnetron suitable for sputtering magnetic targets of sufficient thicknesses to offset the additional cost of the stronger magnets. Unfortunately, however, sputtering magnetrons that magnetically saturate the target suffer from another problem that has proven much more difficult to overcome: Namely, severe magnetic pinching of the plasma.
The magnetic pinching phenomenon is best understood by referring to FIG. 1 which illustrates, in schematic form, the forces acting on electrons in the region of the magnetic tunnel. In a prior art magnetron, a magnet assembly M produces a magnetic field that can be characterized by a plurality of magnetic flux lines F, one of which is shown in FIG. 1. The flux line F shown in FIG. 1 is representative of the field shape immediately adjacent the front surface of the target. Points 1 and 3 are points in the central region of the tunnel, to the left and right of the central axis C respectively. Point 2 is a point on the central axis C of the tunnel, coincident with the flux line F. The target locations within the tunnel fields are such that each of the points 1, 2, and 3 will be coincident with the front surface of the target at some time during the sputtering process. As is well-known, the curvature of the magnetic field F subjects electrons positioned at points 1 and 3 lateral forces F.sub.1 that tend to push them toward the central axis C of the tunnel. Since the magnetic field has no vertical component (i.e., a component orthogonal to the plane of the pole piece) at point 2, no lateral forces are exerted on electrons at point 2. The action of the lateral forces F.sub.1 on the electrons in the plasma tends to force or pinch them toward the central axis C of the magnetic tunnel. Since the erosion caused by sputtering is related to the density of the electrons in the glow discharge plasma, the effect of the pinching phenomenon increases the erosion rate along the central axis C of the magnetic tunnel.
While this pinching phenomenon occurs in all types of planar magnetrons with arched magnetic tunnels, the problem becomes much worse when sputtering magnetic target materials, as best seen in FIGS. 2(a)-(d). When sputtering a magnetic target T, a large portion of the magnetic flux F (shown qualitatively in FIGS. 2(a)-(d)) produced by the magnet assembly M will be shunted through the magnetic target. However, if the magnetic field is strong enough, sufficient magnetic flux will remain over the front surface of the target to produce an arched, plasma-confining magnetic tunnel. As was explained above, the pinching forces resulting from the arched magnetic tunnel will initially pinch the electrons toward the center of the tunnel, resulting in the greatest erosion rate at that point. However, as the target erodes, its cross-sectional area decreases, thus forcing additional magnetic flux from the target. Since the excess magnetic flux always takes the lowest energy path (i.e., the path of least resistance), it usually exits the target surface in the erosion area and re-enters the target just as soon as the cross-sectional area has increased to the point where the target material can again accommodate the excess flux. The liberated magnetic flux arching over the front surface of the target subjects electrons within the glow discharge plasma to even greater pinching forces, which substantially increases the electron density, thus erosion rate, along the center of the tunnel, as best seen in FIG. 2(c). As the target erodes further, more and more magnetic flux is liberated, resulting in stronger pinching forces, higher electron densities, and greater erosion rates. The result is a deep, spike-like erosion groove in the target, as best seen in FIG. 2(d).
The fraction of the target material that has been sputtered away by the time the bottom of the erosion groove reaches the back surface of the target is referred to as the target utilization, and is extremely low for most magnetic targets, in the range of 5%-15% at best. Since most target materials tend to be relatively expensive, such low target utilization is wasteful and increases the costs associated with the sputtering process. For example, although spent targets may be recycled and reworked into new targets, the time spent changing and reworking targets can be significant and in any event, increases the overall cost of the sputtering operation.
Another solution to the problem has been to place external magnets above and around the target to generate the plasma-confining magnetic tunnel. However, such systems are prone to the pinching effect described above, and also add considerable complexity, thus cost, to the magnetron assembly. Moreover, unless properly shielded, the additional magnets themselves may sputter and contaminate the coating.
Yet another approach has been to reduce the strength of the magnetic field required to saturate the target material. Since a magnetic material heated above its Curie temperature loses its ferromagnetism, magnetron sputtering of a ferromagnetic target can be more easily accomplished by heating the target material above its Curie temperature. A disadvantage of this approach is that it requires a device for monitoring the temperature of the target as well as a system for achieving and maintaining the required Curie temperature. Also, since the Curie temperature of most ferromagnetic materials is quite high, in the range of 400.degree. C. to 1100.degree. C., heating the target material to such temperatures can damage the substrate being coated or other parts of the vacuum system. Another disadvantage is that most high performance permanent magnets loose their magnetic properties at temperatures above about 150.degree. C. to 200.degree. C., so a cooling system must be provided to maintain the permanent magnets below the critical temperature.
While still other modifications have been developed, each is not without its problems. For example, Deppisch et al in U.S. Pat. No. 4,652,358, attempt to solve the target thickness problem by placing the ferromagnetic target on a specially configured floor located between the target and the magnet assembly. While most of the floor is made from a non-magnetic material, it includes special ferromagnetic inserts in the regions of the poles of the magnet system to improve the magnetic coupling efficiency, thereby lowering the magnetic field density required to saturate the target. Unfortunately, the floor is difficult and expensive to manufacture and, in the preferred embodiment, requires that the ferromagnetic plugs be joined to the non-magnetic floor by electron beam welding to minimize mechanical stress and to produce gas-tight and liquid-tight joints. Moreover, Deppisch et al do not address the pinching problem, leaving his device with poor target utilization.
The patent issued to Abe et al., U.S. Pat. No. 4,401,539, discloses a method and apparatus for sputtering magnetic target materials that uses a combination of permanent magnets and electromagnets to saturate the target and generate the strong leakage flux density over the target surface required to create the plasma-confining magnetic tunnel. Abe et al attempt to minimize the pinching problem by using the electromagnets to vary the strength and configuration of the magnetic field during the sputtering process. Unfortunately, however, the computer controlled electromagnet assembly is complex and expensive to fabricate.
Boys et al. in U.S. Pat. No. 4,500,409, disclose a planar magnetron for sputtering highly ferromagnetic targets that uses only electromagnets to saturate the target and produce the plasma-confining magnetic field over the target surface. Boys et al. also address the pinching problem by varying the strength of the magnetic field produced by the electromagnets. However, like the magnetron disclosed in the Abe patent, the magnetron disclosed by Boys et al. is relatively complex and expensive to manufacture. Moreover, the design constraints associated with most magnetron cathode assemblies tends to limit the size of the electromagnets and the resulting magnetic field, thus requiring the use of relatively thin targets.
The patents issued to Morrison, U.S. Pat. Nos. 4,391,697 and 4,431,505, disclose yet another type of magnetron sputtering apparatus for sputtering ferromagnetic targets. Morrison's device utilizes a two piece target with a gap between the pieces. According to Morrison, while most of the applied magnetic field concentrates in the gap between the target pieces, there remains sufficient excess flux to form a weak plasma trapping field over the surface of the target. The gap serves as a plasma source, which plasma then migrates to the trapping field to sputter the target. Disadvantageously, the two piece target required by the Morrison device is relatively complex. Also, Morrison recognizes that the gap floor can sputter, thus possibly contaminating the coating or, worse yet, erode through to the magnet assembly. However, Morrison seems to accept those disadvantages as the price to be paid for sputtering ferromagnetic materials.
Finally, Aichert et al. in U.S. Pat. No. 4,572,776, attempt to achieve magnetron sputtering of a magnetic target by using a ferromagnetic pole shoe assembly to assist in producing a plasma-confining tunnel over the surface of the target. Unfortunately, portions of the ferromagnetic pole shoe assembly do sputter, thus requiring that it be made of the same material as the target, or else risk contaminating the coating. However, if the pole shoe is made of the same material as the target, then any cost advantages are lost, and Aichert's device simply becomes another magnetron having a two-piece target. Also, the pole shoe and target assemblies must be positioned with respect to each other at certain specifically defined spacings and tolerances that may be difficult to maintain during the sputtering operation.
In sum, while the foregoing devices represent some improvements, they have usually come at the expense of decreased power efficiency, decreased target replacement intervals, or have required two-piece target configurations or the addition of separate or additional electromagnetic coils, along with apparatus to control the electromagnets. Besides increasing the overall cost of the sputtering device, the addition of large numbers of components into the sputtering chamber may poison the sputtered film with unwanted impurities if suitable precautions are not taken to insure that the additional components themselves do not sputter. Also, few of the prior art references provide an effective solution to the magnetic pinching problem.
Therefore, there remains a need for a planar magnetron that can sputter ferromagnetic targets without the severe disadvantages that result from the magnetic pinching problem. Such a magnetron should also be capable of achieving high target utilization rates, but without the need to resort to complex target configurations, electromagnet assemblies or, worse yet, magnet assemblies containing both electromagnets and permanent magnets. Such a permanent magnet magnetron should also be capable of sputtering a relatively thick ferromagnetic target without having to heat it to its Curie temperature.